As is well known, an amplifier with a class A output stage can not supply an output current greater than a quiescent current of the output stage. If it is desired to supply an output current greater than the quiescent current, a different class of amplifier is required. An amplifier with a class B output stage can supply a large output current with zero quiescent current, but introduces a large crossover distortion. A class AB output stage provides a compromise, enabling the output current to be greater than the quiescent current without excessive distortion.
It is well known to provide a CMOS amplifier with a class AB output stage. Typically, such an amplifier may be a differential amplifier which has a differential input stage, and may be referred to as an operational amplifier or opamp.
It is desirable for a differential amplifier to have a low offset voltage, in order to reduce errors in the amplifier output especially for small differential input voltages. For example, it may be desirable for a CMOS differential amplifier to have an offset voltage of less than about 1 mV.
Many class AB CMOS amplifiers have the disadvantage of complex design involving relatively large numbers of transistors. It is desirable to provide a class AB CMOS amplifier using a relatively small number of transistors, in a simple architecture, in order to reduce the amplifier area and to minimize the number of transistors that can contribute to offset.
“CMOS Switched-Capacitor Filters for a PCM Voice CODEC” by R. Gregorian et al., IEEE Journal of Solid-State Circuits, Vol. SC-14, No. 6, December 1979, pages 970–980 shows in FIG. 11 an example of such an amplifier. A similar example is shown in the book “Introduction To CMOS Op-Amps And Comparators” by R. Gregorian, John Wiley & Sons, 1999, in FIG. 4.15 on page 109.
However, especially with typical variations in manufacturing processes, supply voltages, and temperature (referred to as PVT), in such circuits it is difficult to provide a low and controlled quiescent current. In particular, in these known CMOS amplifiers the quiescent current of the class AB output stage can change by a factor of two or more with changes, resulting from PVT variations, in characteristics such as a threshold voltage of a level-shifting transistor of the amplifier.
Accordingly, there is a need to provide an improved CMOS amplifier.